Adaptable high-performance extrusion head for fused filament fabrication systems

ABSTRACT

An extrusion head for a three-dimensional printer is disclosed including a feed tube, a heater, a cooler, and a bridge. The feed tube can be made of metal and has an inlet for receiving a forwardly driven filament of solid deposition material, an outlet, a downstream portion adjacent to the outlet, an upstream portion upstream from the downstream portion, and an internal passage extending from the inlet to the outlet. The heater is thermally coupled with the downstream portion of the feed tube for heating a filament to provide softened fluid deposition material. The cooler is thermally coupled with the upstream portion and spaced generally axially from the heater to define a generally axially extending gap traversed by the feed tube. The bridge traverses the gap and provides a rigid mechanical connection between the heater and the cooler.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation application of co-pending U.S. patentapplication Ser. No. 17/100,506, filed Nov. 20, 2020, which is acontinuation of U.S. patent application Ser. No. 15/981,615, filed May15, 2018, now U.S. Pat. No. 10,875,244, granted on Dec. 29, 2020, whichclaims the priority of U.S. Provisional Application No. 62/507,728,filed May 17, 2017. The entire content of all of the above-identifiedapplications are incorporated by reference herein.

BACKGROUND OF THE INVENTION

The invention relates to the thermal dispensing head for depositinglayers of solidifying material in a desired pattern to formthree-dimensional physical objects. The modeling material is selectedand its temperature is controlled so that it solidifies upon extrusionfrom the dispensing head onto a base, with the build-up of multiplelayers forming the desired article. This method of fabrication is oftencalled Fused Filament Fabrication (FFF), and the thermal dispensing headfor a FFF machine is often called the hot end.

Examples of apparatus and methods for FFF of three-dimensional objectsby depositing layers of solidifying material are described in Crump U.S.Pat. No. 5,121,329; Batchelder et al. U.S. Pat. No. 5,303,141; CrumpU.S. Pat. No. 5,340,433; Batchelder U.S. Pat. No. 5,402,351; BatchelderU.S. Pat. No. 5,426,722; Crump et al. U.S. Pat. No. 5,503,785; Abrams etal. U.S. Pat. No. 5,587,913; and Swanson et al. U.S. Pat. No. 6,004,124.The systems disclosed in the '329, '433, '785 patents and '124 patents,for example, describe an extrusion head which receives a solid statematerial used to form three dimensional articles, heats the material toabove its solidification temperature, and dispenses the material as afluid onto a base.

Various embodiments of the extrusion head are shown in the Crump '433patent. Each embodiment includes a liquefier which consists of threezones: an entrance zone or cap, a heating zone or body and a nozzle. Afirst embodiment is shown in FIG. 3 of the '433 patent. FIG. 3 shows aliquefier within an extrusion head having a seal ring (i.e., a cap), aheating head (i.e., heating zone) and a nozzle. The seal ring receives asupply rod of solid material. An electric heater within the heating headheats the supply rod to a temperature exceeding its solidificationtemperature, reducing it to a liquid state. The liquid material thenflows into the nozzle through a nozzle flow passage, and is dispensedthrough a nozzle dispensing outlet.

A second embodiment of the extrusion head is shown in FIG. 5 of theCrump '433 patent. In this embodiment, the supply material is in theform of a flexible strand in solid form. The flexible filament ofmaterial shown in FIG. 5 is fed through a guide sleeve to an extrusionhead. The extrusion head contains a supply chamber in a top portion anda liquefier in a bottom portion. Drive rollers within the supply chamberintroduce the flexible strand into the liquefier. The liquefier withinthe extrusion head includes a seal ring (i.e., a cap), a material supplyand flow passage (i.e., heating zone) and a dispensing outlet orifice(i.e., a nozzle). The flexible strand is advanced into the liquefierthrough the seal ring, which provides a hydraulic seal around theinternal surface of the flow passage. A heater in the form of a sleevecontaining a heating coil is positioned around the flow passage and theorifice to heat the strand to a fluid state in the passage. The materialis dispensed in a fluid state through the orifice.

A third embodiment of the extrusion head is shown in FIG. 13 of theCrump '433 patent. As with the embodiment shown in FIG. 5, the materialis supplied in the form of a flexible strand in solid form. The strandis advanced into an extrusion head through a guide sleeve. A strandadvance mechanism comprising a pair of motor-driven feed rollers orpulleys and advances the strand into the liquefier. The liquefier ofFIG. 13 is comprised of a tubular guide member, a seal ring, a liquefiernozzle and a removable tip. The tubular guide member and seal ringtogether form the cap zone. The tubular guide member is made of highlyconductive metal. It dissipates heat rapidly to maintain the flexiblestrand at a suitable temperature during its movement from the strandadvance mechanism into the heating zone. To further dissipate heat fromthe guide member, a blower may be used to circulate air into theextrusion head, around the guide member. At its lower end, the guidemember is supported on the seal ring. The seal ring is made out ofheat-insulating plastic to serve as a thermal seal. The liquefier nozzlesurrounded by a heating coil and an outer insulation sleeve provides aheating zone in which the strand material is melted. The liquefiernozzle (i.e., heating tube) is made of heat-conducting material. Theremovable tip is attached to the bottom end of the liquefier nozzle by athreaded connection.

A fourth embodiment of the extrusion head is shown in FIG. 6 of theCrump '433 patent. In this embodiment, multiple materials are dispensedthrough separate passages into a single discharge outlet. The embodimentof FIG. 6 allows utilization of different materials to form differentlayers of the same article.

The Crump '785 patent discloses an extrusion head carrying twoliquefiers, each having its own nozzle. The liquefiers of the '785patent each have a cap at a receiving end, secured by a mounting ring toa tubular dispenser (i.e., heating tube). A heating coil is wrappedaround each tubular dispenser to heat and melt a filament of material.In each liquefier, the material is provided in a fluid state to adispensing nozzle and discharged through a nozzle tip. Filament isconveyed to each liquefier from a supply spool by a pair of pinchrollers driven by stepper motors.

In the aforementioned liquefiers, the cap region serves as thetransition zone for the modeling material where at the entrance to thecap the temperature is below the softening point of the material and theoutlet of the cap is above the temperature required to pump the materialin a semi-liquid state. This requires a change in temperature of up to250° Celsius over the length of the cap. Ideal properties for the capare a high thermal resistivity in the axial direction and low thermalresistivity in the radial direction. Designs such as those described inthe Crump patents used high temperature thermoplastics or thermosetssuch as Dupont “Vespel” SP-1, for the cap to accomplish these goals.These caps have temperature limitations and require a sealing mechanismbetween the cap and the heating body, which is typically formed ofaluminum. The caps and seal are prone to leakage.

A fifth embodiment of the extrusion head is shown in FIG. 9 of the '124patent. In this embodiment, a liquefier formed of a single piece ofthin-wall tubing is encased in a heating block. The tube acts as boththe hot zone and the cold zone of the liquefier. The nozzle can beformed by swaging the metal tube to a nozzle, or it may be brazed orwelded to the bottom of the tube. The heating block is made of heatconductive materials.

The thin-wall tube has an inlet end for receiving a filament of moldingmaterial and an outlet end for delivering the material in liquid form. Afirst section of the tube adjacent the inlet end functions as theentrance or cap zone. This first section of the tube is exterior to theheating block. The tube has a second section which passes through theheating block forming a heating zone. The nozzle connects to the outletend of the tube. The cap zone of the tube must dissipate heat rapidly tomaintain the flexible strand at a suitable temperature during itsmovement into the heating zone, so that the strand will not become limpand buckle. A stainless steel tube having a wall thickness in the rangeof 0.008-0.015 inches and an interior diameter of 0.07 inches isspecified in the '124 patent.

Example products include: E3D V6, Prusa MKII, Lulzbot Hexagon, DyzeDyzend-X and many others.

SUMMARY OF THE INVENTION

An aspect of the invention is an extrusion head for a three-dimensionalprinter. The extrusion head includes a feed tube, a heater, a cooler,and a bridge.

The feed tube can be made of metal and extends generally axially. Thefeed tube has an inlet for receiving a forwardly driven filament ofsolid deposition material, an outlet, a downstream portion adjacent tothe outlet, an upstream portion upstream from the downstream portion,and an internal passage extending from the inlet to the outlet.

The heater is thermally coupled with the downstream portion of the feedtube for heating a filament positioned within the feed tube internalpassage to provide softened fluid deposition material.

The cooler is thermally coupled with the upstream portion for reducingupstream heat transfer. The cooler is spaced generally axially from theheater to define a generally axially extending gap traversed by the feedtube.

The bridge is spaced radially from the metal feed tube, traverses thegap, and provides a rigid mechanical connection between the heater andthe cooler.

Other aspects of the invention are described or will become apparentfrom the following description and the drawing figures.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIGS. 1, 2, and 3 are perspective views of an embodiment of theextrusion head.

FIG. 4 is a front elevation view of an embodiment of the extrusion headof FIGS. 1 through 3 .

FIG. 5 is a top plan view of an embodiment of the extrusion head ofFIGS. 1 through 3 .

FIG. 6 is a bottom plan view of an embodiment of the extrusion head ofFIGS. 1 through 3 .

FIG. 7 is a side elevation view of an embodiment of the extrusion headof FIGS. 1 through 3 .

FIG. 8 is a section view taken along section lines 8-8 of FIG. 7 of anembodiment of the extrusion head of FIGS. 1 through 3 .

FIG. 9 is a section view taken along section lines 9-9 of FIG. 7 of anembodiment of the extrusion head of FIGS. 1 through 3 .

FIGS. 10 through 12 are perspective views of an embodiment of theextrusion head.

FIG. 13 is a front elevation view of an embodiment of the extrusion headof FIGS. 10 through 12 .

FIG. 14 is a top plan view of an embodiment of the extrusion head ofFIGS. 10 through 12 .

FIG. 15 is a bottom plan view of an embodiment of the extrusion head ofFIGS. 10 through 12 .

FIG. 16 is a side elevation view of an embodiment of the extrusion headof FIGS. 10 through 12 .

FIG. 17 is a section view taken along section lines 17-17 of FIG. 16 ofan embodiment of the extrusion head of FIGS. 10 through 12 .

FIGS. 18 through 20 are perspective views of an embodiment of theextrusion head.

FIG. 21 is a front elevation view of an embodiment of the extrusion headof FIGS. 18 through 20 .

FIG. 22 is a top plan view of an embodiment of the extrusion head ofFIGS. 18 through 20 .

FIG. 23 is a bottom plan view of an embodiment of the extrusion head ofFIGS. 18 through 20 .

FIG. 24 is a side elevation view of an embodiment of the extrusion headof FIGS. 18 through 20 .

FIG. 25 is a section view taken along section lines 25-25 of FIG. 24 ofan embodiment of the extrusion head of FIGS. 18 through 20 .

FIGS. 26 through 28 are perspective views of an embodiment of theextrusion head.

FIG. 29 is a front elevation view of an embodiment of the extrusion headof FIGS. 26 through 28 .

FIG. 30 is a top plan view of an embodiment of the extrusion head ofFIGS. 26 through 28 .

FIG. 31 is a bottom plan view of an embodiment of the extrusion head ofFIGS. 26 through 28 .

FIG. 32 is a side elevation view of an embodiment of the extrusion headof FIGS. 26 through 28 .

FIG. 33 is a section view taken along section lines 33-33 of FIG. 32 ofan embodiment of the extrusion head of FIGS. 26 through 28 .

A list of the reference characters used in the drawings follows.

100 Extrusion Head 101 Cooler 102 Heater 103 Nozzle 104 Feed Tube 105Second Cooler 106 Bushing 107 Spacer 108 Tension Member 109 Inlet (of104) 110 Filament 111 Outlet (of 104) 112 Downstream Portion (of 104)113 Upstream Portion (of 104) 114 Internal Passage (of 104) 115 SoftenedDeposition Material 116 Gap 117 Bridge 118 Platform 119Three-Dimensional Printer 120 Heating Element 121 Temperature Sensor 122Threaded Bore (of 102) 123 External Thread (of 112) 124 Axial Bore (of106) 125 Exterior Threaded Surface (of 106) 126 Inlet (of 122) 127Outlet (of 122) 128 First Thermally Conductive Portion 129 SecondThermally Conductive Portion 130 Thermally Conductive Flange Portion 131Internal Heat Transfer Passage 132 Cooling Fluid 133 Sleeve (Heat Sink)134 First Portion (of 107) 135 Second Portion (of 107)

DETAILED DESCRIPTION OF THE DISCLOSURE

FIGS. 1 to 33 show exemplary extrusion heads 100 for a three-dimensionalprinter or similar device 119 also including a supply of filamentmaterial 110, a part support base 118, and a mechanism, which can beconventional, for moving the extrusion head 100, the building table 118,or both relative to the other. The extrusion head 100 includes, forexample, a cooler 101, a heater 102, a nozzle 103, a feed tube 104, asecond cooler 105, a bushing 106, a spacer 107, and a tension member108.

The feed tube 104 in this embodiment is made of metal, and extendsgenerally axially. The feed tube 104 has an inlet 104 for receiving aforwardly driven filament 110 of solid deposition material, an outlet111, a downstream portion 112 adjacent to the outlet 111, an upstreamportion 113 upstream from the downstream portion 112, and an internalpassage 114 extending from the inlet 104 to the outlet 111.

The heater 102 is thermally coupled with the downstream portion 112 forheating a filament 110 positioned within the feed tube 104 internalpassage 114 to provide softened deposition material 115.

The cooler 101 is thermally coupled with the upstream portion 113 forreducing upstream heat transfer. The cooler 101 is spaced generallyaxially upstream from the heater 102 to define a generally axiallyextending gap 116 traversed by the metal feed tube 104.

A bridge 117 (for example, at least one spacer 107 or at least onetension member 108) is spaced radially from the metal feed tube 104,traversing the gap 116, and providing a rigid mechanical connectionbetween the heater 102 and the cooler 101.

Optionally in any embodiment, the metal feed tube 104 comprisesstainless steel or zirconia, and suitably can be made from hypodermictubing.

Optionally in any embodiment, the hypodermic tubing is sized from 10XXto 14XX gauge.

Optionally in any embodiment, the metal feed tube 104 has a wallthickness from 0.001 to 0.005 in. (0.025 mm to 0.13 mm), a wallthickness less than 0.005 in. (less than 0.13 mm), or from 0.001 to0.004 in. (0.025 mm to 0.1 mm), or from 0.002 to 0.004 in. (0.05 mm to0.1 mm).

Optionally in any embodiment, the metal feed tube 104 has a wallcross-sectional area from 0.002 in² to 0.005 in² (1 mm² to 3 mm²), orfrom 0.0017 in² to 0.004 in² (1.1 to 2.6 mm²).

Optionally in any embodiment, the metal feed tube 104 has an insidediameter from 0.07 in. to 0.13 in. (1.8 mm to 3.3 mm), or from 0.07 in.to 0.11 in. (1.8 mm to 2.8 mm).

Optionally in any embodiment, the metal feed tube 104 has a length from0.5 in. to 3 in. (12 mm. to 76 mm.). Optionally in any embodiment, theportion of the feed tube 104 traversing the gap 116 extends axially from0.03 in. to 3 inches (0.8 mm. to 76 mm.).

Optionally in any embodiment, the metal feed tube 104 internal passage114 is coated internally with a material reducing adhesion of thedeposition material, for example, electroless nickel, an electrolessnickel-boron composite, tungsten disulfide, molybdenum disulfide, boronnitride, diamond-like carbon, zirconium nitride, titanium nitride, or acombination of two or more of these.

Optionally in any embodiment, the heater 102 comprises a heater blockcomprising thermally conductive material, at least one heating element120, and at least one temperature sensor 121 attached to and in thermalcontact with the heater block 102. Optionally in any embodiment, theheater block has an axial length from 0.2 inches to 1.5 inch (5 mm. to38 mm.). The heater block can have a threaded bore 122.

Optionally in any embodiment, the feed tube 104 downstream portion 112has an external thread 123, and the heater block 102 threaded bore 122and the feed tube 104 external thread 123 are engaged to thermallycouple the heater block 102 with the downstream portion 112 of the feedtube 104. Alternatively, the extrusion head 100 of claim 20 includes abushing 106 having an axial bore 124 defined by a wall secured to thefeed tube 104 downstream portion 112, the bushing 106 further comprisingan exterior threaded surface 125 engaged with the heater block 102threaded bore 122. Optionally, the heater block 102 threaded bore 122extends from an inlet 126 communicating with the feed tube downstreamportion to an outlet 127.

Optionally in any embodiment, the extrusion head 100 includes a nozzle103 secured to the heater block 102 threaded bore 122 and communicatingwith the outlet 127 of the heater block 102 threaded bore 122.

Optionally in any embodiment, the cooler 101 comprises a thermoelectriccooler or a heat sink comprising heat-conductive material. Optionally inany embodiment, the heat sink has at least a first thermally conductiveportion 128 thermally coupled with the upstream portion 113 of the feedtube 104 and a second thermally conductive portion 129 generallyradially spaced from the upstream portion 113 of the feed tube 104.Optionally in any embodiment, the heat sink has a thermally conductiveflange portion 130 extending generally axially from the second thermallyconductive portion 129 and parallel to and radially spaced from the feedtube 104. Optionally in any embodiment, the heat sink has at least firstand second thermally conductive flange portions 130, each extendinggenerally axially from the second thermally conductive portion 129,parallel to and radially spaced from the feed tube 104, and the firstthermally conductive flange portion 130 circumferentially spaced fromthe second thermally conductive flange portion 130.

Optionally in any embodiment, the heat sink comprises an internal heattransfer passage 131 configured to receive a cooling fluid 132.

Optionally in any embodiment, the heat sink comprises a bore in thermalcontact with the feed tube 104 along at least a portion of the gap 116.

Optionally in any embodiment, the bridge 117 comprises a generallyaxially extending spacer 107, spaced radially from the feed tube 104.Optionally in any embodiment, the spacer 107 has at least a firstportion 134 bearing against the heater 102 and a second portion 135bearing against the cooler 101. Optionally in any embodiment, the bridge117 comprises first and second generally axially extending spacers 107,each spaced radially from the feed tube 104, each having at least afirst portion 134 bearing against the heater 102 and a second portion135 bearing against the cooler 101. Optionally in any embodiment, theextrusion head 100 has a third generally axially extending spacer 107,spaced radially from the feed tube 104, and having at least a firstportion 134 bearing against the heater 102 and a second portion 135bearing against the cooler 101. Optionally in any embodiment, theextrusion head 100 has a fourth generally axially extending spacer 107,spaced radially from the feed tube 104, and having at least a firstportion 134 bearing against the heater 102 and a second portion 135bearing against the cooler 101.

Optionally in any embodiment, the spacer 107 at least partially reducesmechanical loading on the feed tube 104.

Optionally in any embodiment, the spacer 107 comprises stainless steel,zirconia, or a combination of stainless steel and zirconia, for examplehypodermic tubing. Examplary suitable hypodermic tubing is sized between7XX and 14XX gauge, inclusive, for example, 7XX, 8XXX, 8XX, 9XXX, 9XX,10XX, 11XX, 12XX, 13XX, 14XX, or a combination of two or more of these.Optionally in any embodiment, the spacer 107 comprises thermalinsulation material, for example, calcium silicate, ceramic, glass, anengineering thermoplastic, zirconia, mica, Portland cement or acombination of any two or more of these.

Optionally in any embodiment, the extrusion head 100 further comprisesat least a first tension member 108 spaced radially from the feed tube104 and connected to and exerting tension between the heater 102 and thecooler 101. Optionally second, third, or fourth tension members 108 canbe provided.

Optionally in any embodiment, the total cross-sectional area of thetension members 108 and spacers 107 is less than 0.01 square inches (6.4mm.²). Optionally, the sum of the contact areas of the tension membersand spacers with the heater is between 0.005 in.² and 0.02 in.² (0.25mm.² and 3.2 mm.²) and with the cooler is between 0.005 in.² and 0.02in.² (0.25 mm.² and 3.2 mm.²).

The inventor contemplates two design tradeoffs inherent in existingall-metal extrusion head designs:

First, the heat break's thermal isolation performance is proportional tothe length of and inversely proportional to the wall thickness of thethin-walled section. Poor thermal isolation results in filamentsoftening prematurely and reduction in print quality alluded to in the'124 patent. The requirement for the heat break to carry a mechanicalload is therefore at odds with its performance. The extrusion headdesigner must select the heat break's wall thickness to withstandreasonable incidental loads caused by machine crashes, failed prints, orhuman mishandling. In this manner the structural requirement put on theheat break hinders performance of the extrusion head, which in turnhinders the overall performance of the FFF machine.

Second, the hot end designer may lengthen the hot zone and select anozzle of large bore diameter to maximize potential speed of printing,or he may shorten the hot zone and select a small-bore nozzle tomaximize printing resolution. Additionally, some extrusion head designsallow users to affect the length of the hot zone by swapping or addingcomponents. Example products: E3D V6-to-Volcano conversion kits, DisTechPrometheus V2. In all such products the overall length of the extrusionhead changes when the user affects the hot zone's length, which is anundesirable side effect. A change in overall length of the extrusionhead requires the user to calibrate the machine's recorded offset fromnozzle to print bed. Failure to perform said calibration results in afailed print or the nozzle crashing into the print bed.

Existing all-metal extrusion heads borrow many design features from the'124 patent, and they all utilize a component known as a heat break tothermally isolate the heater block from cold components. The heat breaktypically:

-   -   a. Consists of a cannulated threaded rod with two threaded        sections separated by a thin-walled section several millimeters        in length,    -   b. Features a thin-walled section with inner diameter and wall        thickness typically in the range specified by the '124 patent        for the thin-walled tube,    -   c. Is made of stainless steel,    -   d. Connects to a finned heatsink or a liquid cooling system, and    -   e. Is the only component connecting the heater block to cold        components. I.e. the heat break not only functions as a thermal        isolator but also as the mechanical structure carrying the        heater block.

In the present improved extrusion head for FFF systems, the liquefiercomponent can be formed of a single piece of thin-wall tubing pressed,brazed, or welded to a bushing of varying length. The thin-walled tubeacts as both the hot zone and the cold zone of the liquefier. The inletof the thin-walled tube slip-fits into a hole in the cold section. Thethin-walled tube can be swaged, brazed or welded to a bushing in thermalcontact with the heating block. The inlet of a removable nozzle can sealwith the outlet of the bushing. The heating block and bushing are madeof heat conductive materials, such as aluminum alloys or copper alloys,preferably a chromium copper alloy due to its combination of thermalconductivity and high strength at the highest temperature rangescommonly encountered in FFF applications.

Unlike other all-metal hot ends, the thin-walled tube does not need tobe a structural member. Nor does the tube need to cantilever from anexternally supported heater block as described in the '124 patent. Sinceit optionally can be partially or completely relieved of mechanicalloading, the wall thickness of the tube can be greatly reduced toimprove its thermal isolation performance. The tube's wall thickness isin the range of 0.001 to 0.005 inches. The tube thickness used inprototypes has been 0.003 inches, in the form of commercially available14XX gauge hypodermic tubing made of stainless steel. Such a drasticreduction in the heat break's wall thickness optionally removes the needfor a finned heatsink component or liquid cooling system and reduces thefan size needed to keep the cold zone cool. The overall length and girthof the extrusion head may be reduced, conserving valuable space in atypically crowded area of the FFF system, and the overall mass may bereduced.

Bushings of varying length may be user-installed to effectively shortenor lengthen the hot zone, to affect the speed/resolution trade offdescribed above. In arrangement employed in the present invention, thebushing extends upward in the direction of the cold zone rather thandown below the heater block. In this manner, bushings of various lengthsmay be used without affecting the overall length of the hot end,preserving the recorded offset to the print bed, and preventing the needfor the user to recalibrate the machine after making adjustments.

The heater block optionally connects to cold zone components via two tofour standoffs and zero to four screws. Optionally, three screws withthree standoffs or the preferable two screws with four standoffs can beused. The standoffs are preferably made of thin-walled tubes orsmall-diameter rods, and the screws are of small cross-sectional area.The standoffs may be constructed off blocks of rigid insulationmaterials such as calcium silicate based materials. Preferably thestandoffs and screws incorporate materials with a high ratio of strengthto thermal conductivity, such as stainless steel or zirconia. Thestructural components connecting the cold and hot zones are loaded onlyin compression (standoffs) and tension (screws) to resist therapidly-changing axial push-pull forces applied by the filament feedsystem. Components between hot and cold zones are not loaded in bending,providing maximum axial rigidity for a given axial cross-sectional areaof the standoff components. The total cross-sectional area of thestandoff structure optionally is minimized to minimize the heat flowingfrom the heater block to cold zone components. For all prototypes of thepresent invention, this cross-section was less than 0.01 square inchesin area and the structure consisted of stainless steel screws andtubular standoffs.

Optionally, the cold section is composed of a hollow heatsink componentof a basically square outer shape, with inward-facing slits for heatdissipation by convection. This component's nominal wall thicknessexcluding the slits is roughly one fifth the overall width of the squarehollow component, and this component is made of aluminum alloy. Abovethis component an adapter is attached to guide the filament from thefeed system into the thin-walled feed tube. The ideal geometry for thisadapter is specific to the FFF system. Use of an adapter allows theextrusion head to be installed on a wide variety of makes and models ofFFF systems. Since the adapter is located at the coldest region of theextrusion head, it need not be made of metal. Users are free to designand make their own adapters via FFF or any manufacturing methodconvenient to them.

Optionally, commercially available stainless steel hypodermic tubing isused for the standoffs. Four of these standoffs lightly press intomating counter bores in the cold section and in the heater block. A pairof M1.4×0.3 screws pulls the heater block toward the cold section,establishing the compressive forces in the standoffs. The tubeoptionally is pressed into the bushing, which optionally threads intothe heater block.

Optionally, a heatsink is pressed onto the thin-walled tube. Performanceis not noticeably affected by omission of this heatsink.

Optionally, the heater block is made of chromium copper (aka C182) andis coated with Cerakote Glacier Series ceramic coating. The coatingreduces heat lost via convection and radiation. Electroless nickelplating would also work well due to its low thermal emissivity.

I claim:
 1. An extrusion head for a three-dimensional printer, theextrusion head comprising: a generally axially extending metal feed tubehaving an inlet for receiving a forwardly driven filament of soliddeposition material, an outlet, a downstream portion adjacent to theoutlet, an upstream portion upstream from the downstream portion, and aninternal passage extending from the inlet to the outlet; a heaterthermally coupled with the downstream portion for heating a filamentpositioned within the feed tube internal passage to provide softeneddeposition material; a cooler thermally coupled with the upstreamportion for reducing upstream heat transfer, the cooler spaced generallyaxially upstream from the heater; and a generally axially extending gap,bound by a bridge traversing the gap between the cooler and the heater;wherein, the gap is traversed by the metal feed tube; the bridge isspaced radially and apart from the metal feed tube, the bridge providesa rigid mechanical connection between the heater and the cooler, and thebridge at least partially reduces mechanical loading on the feed tube,wherein the bridge comprises: a structural component, spaced radiallyand apart from the feed tube, and having a first portion bearing againstthe heater and a second portion bearing against the cooler, wherein thestructural component at least partially relieves mechanical loading onthe feed tube.
 2. The extrusion head of claim 1, wherein the structuralcomponent is a spacer or tension member.
 3. The extrusion head of claim1, wherein the bridge further comprises a first structural component anda second structural component, each spaced radially and apart from thefeed tube, and having a first portion bearing against the heater and asecond portion bearing against the cooler, wherein the first structuralcomponent and the second structural component at least partiallyrelieves mechanical loading on the feed tube.
 4. The extrusion head ofclaim 1, wherein the metal feed tube comprises stainless steel.
 5. Theextrusion head of claim 4, wherein the metal feed tube compriseshypodermic tubing.
 6. The extrusion head of claim 1, wherein the metalfeed tube has a wall thickness less than 0.005 in. (less than 0.13 mm).7. The extrusion head of claim 1, wherein the metal feed tube has aninside diameter from 0.07 in. to 0.13 in. (1.8 mm to 3.3 mm).
 8. Theextrusion head of claim 1, wherein the metal feed tube has a length from0.5 in. to 3 in. (12 mm. to 76 mm.).
 9. The extrusion head of claim 1,wherein the portion of the feed tube traversing the gap extends axiallyfrom 0.03 in. to 3 inches (0.8 mm. to 76 mm.).
 10. The extrusion head ofclaim 1, wherein the metal feed tube internal passage is coatedinternally with a material reducing adhesion of the deposition material.11. The extrusion head of claim 10, wherein the material reducingadhesion of the deposition material is electroless nickel, anelectroless nickel-boron composite, tungsten disulfide, molybdenumdisulfide, boron nitride, diamond-like carbon, zirconium nitride,titanium nitride, or a combination of two or more of these.
 12. Theextrusion head of claim 1, further comprising a bushing having an axialbore defined by a wall secured to the feed tube downstream portion, thebushing further comprising an exterior threaded surface engaged with theheater block threaded bore.
 13. (canceled)
 14. The extrusion head ofclaim 1, wherein the structural component is loaded in eithercompression or tension to resist the axial push-pull forces applied bythe filament feed system.
 15. The extrusion head of claim 1, wherein thecooler comprises a heat sink comprising heat-conductive material. 16.The extrusion head of claim 1, wherein the cooler comprises a heat sinkcomprising heat-conductive material and comprising an internal heattransfer passage configured to receive a cooling fluid.
 17. Theextrusion head of claim 1, wherein the structural component compriseshypodermic tubing.
 18. The extrusion head of claim 17, wherein thehypodermic tubing is sized between 7XX and 14XX gauge, inclusive. 19.The extrusion head of claim 1, wherein the structural componentcomprises thermal insulation material.
 20. The extrusion head of claim19, wherein the thermal insulation material is calcium silicate,ceramic, glass, an engineering thermoplastic, zirconia, mica, Portlandcement or a combination of any two or more of these.